The 555 Monostable

Posted on Feb 6, 2014

The 555 timer is so named because its primary mode of operation is intended to be in monostable mode. Operating as a monostable, it does not fit the strict definition of an oscillator because, unlike true oscillators, it requires an input signal to trigger its operation, however the fact that the 555 timer can be used in both monostable and astabl

The 555 Monostable
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e mode considerably increases its flexibility and usefulness. Unlike the astable, which has two unstable states and so continually switches from one to the other and back again, the monstable has one stable state and one unstable state. When triggered by a suitable pulse at its input (pin 2) it switches from its stable state, in which the output is low, to its unstable state where its output is high. This state exists for a time controlled by the values of R1 and C1, and at the end of this period the output switches back to its stable (low) state. Its primary use is therefore to produce a set time delay, initiated by an input pulse. From Fig. 4. 5. 1 it can be seen that the circuit differs from the basic astable configuration shown in Oscillators Module 4. 3 in that only one timing resistor (R1) is used, pins 6 and 7 (instead of 6 and 2) are connected together and pin 2 is used for the trigger pulse input. Fig. 4. 5. 2 illustrates the timing waveforms for the monostable, notice that the trigger pulse on pin 2, which must be higher than 1/3Vcc in the absence of a trigger pulse but is normally at about +Vcc, falls to less than 1/3 Vcc to trigger the start of the delay (high output) period. The trigger pulse makes the voltage on the inverting input of comparator 2 lower than its non-inverting input and so the comparator output goes high, making the S input of the bistable high and setting the bistable Q output high, and its Q...

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